Tyre pressure can be quite a thing in the sticks here.
Some people swear by the 4psi the rule.
Now, I don't know whether it applies to bias plies. It may have come about in the time of radials. Anyway, this is what Coopers say:
"As a general rule, the following can be used - for road use only: For passenger tyres, inflate your cold tyres to the recommended tyre pressure on your vehicle’s tyre placard*. Then to determine if you have the correct pressure for a given load, note the cold pressure reading. Drive for at least 20-30 minutes to ensure they have reached operating temperature and then check again. Ideally, they should be about 4psi above the cold pressure. If the pressure is more than 4psi above the cold pressure, you should add more air. That is because there is too much friction, which builds up more heat than desirable. Conversely, if they are less than 4psi above cold pressure, the cold pressure is too high. Adjust your pressures accordingly."
https://www.coopertires.com.au/media...-pressures.pdf